System and method for generating a combined model for isothermal and anisothermal fatigue life

Abstract

To generate a combined model for isothermal and anisothermal fatigue life of a material, multiple strain-controlled fatigue tests are performed on the material. For each test, test data is generated that includes a normalized load and a number of cycles to occurrence of crack initiation in the material under a normalized load which is a function of multiple instantaneous load levels determined at different points in time during the test. Each individual instantaneous load level is determined by normalizing a measured stress at an instant with a value of a temperature dependent property of the material corresponding to a temperature of the test at that instant. The test data from the plurality of strain-controlled tests are processed to generate a combined lifetime model defining a response of the number of cycles to occurrence of crack initiation to the normalized load.

1 . A method for generating a combined model for isothermal and anisothermal fatigue life of a material subject to cyclic loading, the method comprising:
 performing a plurality of strain-controlled fatigue tests on the material;   generating test data for performance of each strain-controlled test by:   determining a normalized load on the material, wherein the normalized load is a function of a plurality of instantaneous load levels determined at different points in time during the test, wherein each instantaneous load level is determined by normalizing a measured instantaneous stress at a respective point in time with a temperature dependent property of the material wherein the temperature dependent property has a value which corresponds to an instantaneous temperature of the test at the respective point in time;   measuring a number of cycles to occurrence of crack initiation in the material corresponding to the normalized load;   wherein the test data comprises the determined normalized load and the measured number of cycles to occurrence of crack initiation; and   processing the test data generated from the plurality of strain-controlled tests to generate a combined lifetime model for the material defining a response of the number of cycles to occurrence of crack initiation to the normalized load.   
     
     
         2 . The method according to  claim 1 , wherein at least one of the tests is an isothermal LCF test, and wherein:
 the plurality of instantaneous load levels comprises a first load level and a second load level;   determining the first load level by normalizing a maximum measured instantaneous stress on the material with a value of the temperature dependent property of the material corresponding to the temperature of the test; and   determining the second load level by normalizing a minimum measured instantaneous stress on the material in the test with the value of the temperature dependent property of the material corresponding to the temperature of the test.   
     
     
         3 . The method according to  claim 2 , wherein all of the tests are isothermal LCF tests, and each of the tests is carried out at a respective different temperature. 
     
     
         4 . The method according to  claim 1 , wherein at least one of the tests is an anisothermal TMF test, and wherein:
 the plurality of instantaneous load levels comprises a first load level and a second load level;   determining the first load level by normalizing a measured instantaneous stress at a maximum temperature of the test with a value of the temperature dependent property of the material corresponding to the maximum temperature; and   determining the second load level by normalizing a measured instantaneous stress at a minimum temperature of the test with a value of the temperature dependent property of the material corresponding to the minimum temperature.   
     
     
         5 . The method according to  claim 1 , wherein the temperature dependent property is an ultimate tensile strength of the material. 
     
     
         6 . The method according to  claim 4 , wherein the temperature dependent property is an ultimate tensile strength of the material and wherein the normalized load for each strain-controlled test is determined on the basis of a relationship defined by
     P =α|σ( T   min )/UTS( T   min )| b   +c|σ ( T   max )UTS( T   max )| d ,|
   wherein   P denotes the normalized load,   T max  and T min  respectively denote the maximum and the minimum temperature of the test,   UTS(Tm a x) and UTS(T m ± n ) respectively denote the ultimate tensile stress of the material at the maximum and at the minimum temperature,   o(T max ) and σ(T m ± n ) respectively denote the measured stress on the material at the maximum and at the minimum temperature of the test, and   a r  b r  c and d are weighing parameters greater than zero.   
     
     
         7 . The method according to  claim 1 , further generating a combined lifetime model using a sigmoid model defined by 
       
         
           
             
               
                 P 
                 = 
                 
                   
                     A 
                     
                       1 
                       + 
                       
                         ( 
                         
                           N 
                           B 
                         
                         ) 
                       
                     
                   
                   + 
                   D 
                 
               
               , 
             
           
         
         wherein 
         P denotes the normalized load, 
         N denotes the number of cycles to occurrence of crack initiation, and 
         A, B, C r  D are model parameters. 
       
     
     
         8 . The method according to  claim 1 , further comprising feeding the test data of the plurality of strain-controlled tests to a modeling device, and performing the processing of the test data to generate the combined lifetime model by the modeling device. 
     
     
         9 . A method for estimating fatigue life of a component operable under cyclic loading, comprising:
 determining instantaneous operational temperatures and corresponding instantaneous operational stresses on the component for a plurality of operational instants;   determining a normalized operational load on the component, as a function of a plurality of instantaneous operational load levels determined for different operational instants, wherein each instantaneous operational load level is determined by normalizing an instantaneous operational stress on the component, as determined for a respective operational instant, with a temperature dependent material property of the component wherein the temperature dependent material property has a value which corresponds to an instantaneous temperature as determined for the respective operational instant; and   providing the determined normalized operational load to a fatigue life estimation device, for determining an estimated number of cycles to occurrence of crack initiation in the component on the basis of a combined lifetime model corresponding to the material of the component, and generating the combined lifetime model by the method according to  claim 1 .   
     
     
         10 . The method according to  claim 9 , wherein the component is a component of a gas turbine, and wherein the instantaneous operational temperatures and the corresponding instantaneous operational stresses are determined by a computerized simulation of an operation of the gas turbine. 
     
     
         11 . A method for operating a component under cyclic loading, comprising:
 scheduling a downtime or maintenance interval of the component taking into account an estimated fatigue life of the component, and determining the estimated fatigue life by a method according to  claim 9 .   
     
     
         12 . A system for generating a combined model for isothermal and anisothermal fatigue life of a material subject to cyclic loading, the system comprising:
 a testing unit configured and operable for performing a plurality of strain-controlled fatigue tests on the material, the testing unit comprising:   a load determining device for determining a normalized load on the material for each test, wherein the normalized load is a function of a plurality of instantaneous load levels determined at different points in time during the test, and wherein each instantaneous load level is determined by normalizing a measured instantaneous stress at a respective point in time with a temperature dependent property of the material wherein the temperature dependent material property has a value which corresponds to an instantaneous temperature of the test at the respective point in time;   a measurement device for measuring a number of cycles to occurrence of crack initiation in the material corresponding to the normalized load of each test;   wherein the testing unit is configured to generate test data comprising the determined normalized load and the measured number of cycles to occurrence of crack initiation; and   a modeling device configured and operable for processing the test data generated from the plurality of strain-controlled tests for generating a combined lifetime model defining a response of the number of cycles to occurrence of crack initiation to the normalized load.